GCSE Electronics: Oscilloscope and Sound Waves

Voltage Gain and Bandwidth

• The voltage gain of an amplifier is closely linked to its bandwidth, with a higher voltage gain resulting in a smaller bandwidth. This is because a higher gain indicates a more selective amplification, which rejects signals outside of the desired frequency range, thereby reducing the bandwidth.

• The gain-bandwidth product is a key concept, where the product of the gain and bandwidth is constant for a given amplifier. This means that if the gain of an amplifier is increased, its bandwidth will decrease, and vice versa. This concept is crucial in the design of amplifiers, as it allows engineers to optimize the trade-off between gain and bandwidth.

Graph of Voltage Gain vs Frequency

• The graph of voltage gain vs frequency shows a decrease in gain from the maximum gain, with the bandwidth being the range of frequencies that can be amplified to more than 70% of the maximum gain. This graph provides a visual representation of the gain-bandwidth trade-off, allowing designers to easily identify the optimal operating point for their amplifier.

• The graph can be used to estimate the bandwidth of an amplifier by drawing a horizontal line at 70% of the maximum gain and reading off the frequency at which the line intersects with the graph. This method provides a quick and accurate way to determine the bandwidth of an amplifier, without requiring complex calculations or simulations.

Examples of Voltage Gain and Bandwidth

• Example 1: An amplifier has a maximum gain of 400 and a bandwidth of 25 kHz when the gain is 280. In this example, the amplifier is designed to operate at a gain of 280, which corresponds to a bandwidth of 25 kHz. This means that the amplifier is capable of amplifying frequencies up to 25 kHz with an amplitude of at least 70% of the maximum gain.

• Example 2: A pre-amplifier with a voltage gain of 200 has a bandwidth of 20 kHz, and the gain x bandwidth product is 1 MHz. In this example, the pre-amplifier is designed to operate at a gain of 200, which corresponds to a bandwidth of 20 kHz. Additionally, the product of the gain and bandwidth is 1 MHz, indicating that the amplifier is capable of amplifying signals up to 1 MHz.

Amplitude and Frequency of AC Signals

• An AC signal contains both positive and negative voltages, and its amplitude is defined as the maximum 'height' of the positive part of the wave. The amplitude of an AC signal determines the magnitude of the voltage variation, and is a critical parameter in the design of amplifiers.

• The frequency of an AC signal can also change, and amplifiers must boost the amplitude of the signal while leaving the frequency unchanged. This means that amplifiers must be designed to operate over a specific range of frequencies, and must be able to handle signals with varying amplitudes and frequencies.

Designing Amplifiers

• Amplifiers are designed to accept a specific range of frequencies, and the range of frequencies that can be amplified correctly is defined as the bandwidth of the amplifier. To design an amplifier, engineers must consider the desired frequency range, the gain required, and the noise and distortion limits.

• Different amplifiers are designed for different frequency ranges, such as telephone amplifiers (300 Hz to 3 kHz) and music amplifiers (20 Hz to 20 kHz). Each amplifier is optimized for its specific application, taking into account the characteristics of the signals to be amplified.

Exercise 5.3

• Exercise 5.3 provides practice questions for calculating the bandwidth of an amplifier based on its gain and gain-bandwidth product. These questions allow students to apply the concepts learned in this section to real-world problems, and to develop a deeper understanding of the relationship between gain, bandwidth, and frequency response.